GSA's Top Geoscience Journal Posts Nine New Articles

Geology articles posted online 29 April 2013

Boulder, Colorado, USA –New GEOLOGY papers cover ancient iron oceans; the Antarctic and global climate/carbon-cycle feedbacks; evidence of catastrophic spillover from kilometer-deep bodies of water on Mars; the role of volcanic emissions in ozone depletion; "fingerprinting" San Andreas fault sandstone; a climax in Earth's mountain-building cycle; the last place on land undergoing continental breakup; garnet as a proxy for dehydration in subduction zones; and evidence of migrating mammals at the Venta del Moro fossil site, Spain.

GEOLOGY articles published ahead of print can be accessed online at http://geology.gsapubs.org/content/early/recent. All abstracts are open-access at http://geology.gsapubs.org/. Representatives of the media may obtain complimentary GEOLOGY articles by contacting Kea Giles at the address above. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in articles published. Contact Kea Giles for additional information or assistance. Detailed highlights are provided below.

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**********Stability of the nitrogen cycle during development of sulfidic water in the redox-stratified late Paleoproterozoic Ocean
Linda V. Godfrey et al., Institute of Marine and Coastal Sciences, Rutgers University, New Brunswick, New Jersey 08901, USA. Posted online ahead of print on 29 April 2013; http://dx.doi.org/10.1130/G33930.1.

Around 1,840 million years ago, a widespread transition to euxinia occurred along productive continental margins. Anoxic sulﬁdic conditions extended outward from near-shore to the mid-shelf, and were overlain by oxic surface waters and underlain by anoxic and Fe-rich (ferruginous) water maintaining the redox stratiﬁcation of the ocean, which developed with the evolution of oxygenic photosynthesis. Widespread sulﬁdic conditions throughout the Mesoproterozoic have been implicated in the protracted oxygenation of the atmosphere and slow rates of eukaryotic evolution. However, the continuation of ferruginous deep water conditions through the Mesoproterozoic, with sulﬁdic conditions limited to areas of high organic carbon production, highlights that the controls on productivity, and oxygenic productivity in particular, remain poorly understood. Linda Godfrey and colleagues report new nitrogen and carbon isotope data for sediments from six drill cores that provide a 350-km-long oblique transect to the paleo-coastline of the Animikie Basin on the margin of Superior Province, North America.

Evidence has emerged confirming a key role for Antarctic regional climate changes in regulating atmospheric CO2 on millennial time-scales. Using geochemical evidence from a sub-Antarctic marine sediment core, geoscientists reveal that the two pulses in atmospheric CO2 that occurred during the last deglaciation (~20,000 to 10,000 years ago) coincided with a surge in the rate at which carbon was exchanged between the atmosphere and the sub-surface Southern Ocean. While the glacial-interglacial climate cycles of the late Pleistocene (the last ~two million years) were paced by gradual changes in the seasonality of solar radiation (i.e., insolation), ultimately they were driven and amplified by strong positive feedbacks within the climate system, including changes in atmospheric CO2 in particular. In this study, L.C. Skinner and colleagues show that this deglacial pulse in ocean ventilation was not driven by the North Atlantic overturning alone, and must have involved an increase in the ventilation of southern-sourced deep waters. Their results thus confirm the removal of a physical and/or dynamical barrier to effective air-sea (CO2) exchange in the Southern Ocean during deglaciation and highlight the Antarctic region as a key locus for global climate/carbon-cycle feedbacks.

The occurrence of Hesperian age (3.7–3.4 billion years old) standing bodies of water at the surface of Mars is a contentious issue, often conflicting with favored climate models. Extensive lakes are proposed to have filled parts of Valles Marineris during this period, yet evidence for their presence and temporal continuity is poorly constrained. In this study, Nicholas Warner and colleagues test the lake hypothesis within eastern Valles Marineris using a new suite of high-resolution imagery and topography data. They analyze the complex geomorphologic and chronologic relationships between structurally controlled, chaotic terrain basins and their associated catastrophic outﬂow channels. The study results demonstrate that the outﬂow channels and interbasin chasmata exhibit evidence for signiﬁcant bedrock erosion by catastrophic spillover from preexisting, kilometer-deep bodies of water.

Volcanoes of the Nicaraguan Volcanic Arc have produced 14 Plinian eruptions within the last 70,000 years. Their eruption columns and therefore released volatiles reached well into the stratosphere, where they had potential to influence atmospheric chemistry and climate. While previous research has focused on the sulfur and chlorine emissions during such large eruptions, S. Kutterolf and colleagues here present measurements of the heavy halogen bromine by means of synchrotron radiation induced micro-XRF microanalysis. Spot analyses of pre-eruptive glass inclusions trapped in minerals (0.5 to 13 ppm) were compared with those in matrix glasses of the tephras (0 to 2 ppm), which represent the post-eruptive, degassed concentrations. The concentration difference between inclusions and matrix glasses, multiplied by erupted magma mass yields estimates of 4 to 600 kilotons of degassed bromine. With respect to an assumed pre-1980 mean stratospheric background and a conservative estimation that only 10% of the halogens per eruptions reached the stratosphere additionally an average of about two times the normal global annual halogen contamination have been added per Nicaraguan Plinian eruption in the past. These results stress the importance of considering volcanic Br emissions along with Cl and sulfate aerosol in order to assess the naturally induced ozone depletion in the stratosphere and their harmful ultraviolet radiation affecting the Earth surface.

The San Andreas fault system forms the boundary between the Pacific and North American tectonic plates in California (United States). Researchers agree that this plate boundary developed about 27 million years ago and that about 315 km of horizontal offset has occurred in the Neogene period (23 million years ago to the present day). Researchers also think that the San Andreas fault was inactive between about 23 and 50 million years ago, based on the correlation of two sandstone formations that were deposited in a deep-ocean basin about 40 million years ago. Glenn R. Sharman and colleagues tested this correlation by “fingerprinting” these sandstone formations based on the radiometric age of individual sand grains of the mineral zircon. They found that the ages of zircon grains were different on either side of the San Andreas fault, suggesting these sandstones were derived from different regions and were never correlative. Sharman and colleagues propose an alternative tectonic reconstruction that implies that at least 50 to 75 km of the previously unrecognized offset occurred between about 23 and 38 million years ago along the San Andreas fault or a predecessor fault. This finding has important implications for how geoscientists understand the tectonic evolution of the western margin of North America.

Analysis of global geochemical datasets obtained through analyses of dated zircon grains show that Earth experienced a peak in the degree of crustal recycling about 1.2 to 1.1 billion years ago, during the Grenvillian Superevent that resulted in amalgamation of supercontinent Rodinia. That the high values were never reached prior or subsequent to this event suggests that this interval represents a climax in Earth's mountain-building cycle, which Martin Van Kranendonk and colleagues postulate arose from a Goldilocks (just right) combination of large plates on a warmer, more rapidly convecting Earth. Prior to the Grenvillian superevent, plates were smaller and less rigid, precluding high degrees of crustal recycling, whereas continued cooling after the Grenvillian superevent resulted in slower drift rates and lower orogenic intensity.

Plate tectonics continually reshape Earth's surface, breaking apart continents over millions of years. In this study, J.O.S. Hammond and colleagues use seismology to image beneath the Afar depression, the northern extreme of the East Africa rift and the only place on land undergoing the final stages of continental breakup. They record distant earthquakes on seismometers deployed across East Africa and, much like a doctor uses X-rays, use this energy to build 3-D images of Earth structure to depths of 400 km. These tomographic images show that sharp changes in rift orientation at the surface are mimicked 75 km deep in the mantle. Hammond and colleagues show that narrow hot zones and regions of upwelling material beneath the stretched and thinned rift zone produce melt, supplying volcanoes in the region and feeding the formation of new oceanic crust in Afar.

The progressive dehydration of subducted rocks represents a fundamental part of the global geologic water cycle. This release of water is a crucial triggering process for arc volcanoes, earthquakes, and the growth and maturation of continents. While models exist to predict water release from the subducting oceanic slab, it is challenging to reconstruct and test these fluid fluxes in nature without a marker in the dehydrated rock residue that can be linked to the primary fluid production. Here, Ethan Baxter and Mark Caddick show that the growth of garnet may be used as a proxy for dehydration in subduction zones. Dehydrating minerals such as lawsonite, chlorite, amphibole, and epidote contribute to garnet growth, especially between ~1.4-3.0 GPa pressures where geophysical models and observations predict dehydration. Thermodynamic analysis of garnet-forming dehydration reactions permits quantification of the average water:garnet production ratio during subduction for varying rock types. Garnet abundance, its pressure-temperature growth span, and its growth chronology may be used to recognize, reconstruct, and test models for progressive subduction zone dehydration.

African camels and rodents migrated to Iberia, and European rabbits and rats migrated to North Africa at the same time before isolation of the Mediterranean Sea during the Messinian. A new date of 6.23 million years ago for the Venta del Moro fossil site (Spain) can be regarded as the first appearance datum for the African migrants Paraethomys and Paracamelus in Europe. At the same time (6.21 million years ago), the North African site of Afoud-1 (Morocco) shows the first Messinian European migrants (e.g. Prolagus, Apodemus). This indicates that 0.25 million years before the onset of the Messinian Salinity Crisis (MSC), a mammal exchange occurred between Europe and Africa through an ephemeral land-bridge. Intensification of the last Miocene glaciations 6.26 million years ago and associated glacioeustatic sea level falls facilitated this corridor allowing a limited exchange of immigrants and serving as a prelude of a new Afro-Iberian dispersal that would follow during the desiccation of the Mediterranean Sea (5.5 million years ago) during the MSC. At 5.33 million years ago, the Zanclean flood refilled the Mediterranean basin and originated the strait of Gibraltar, a marine barrier that impeded additional dispersals during the remainder of the Neogene.